The invention relates to a conveyor assembly, commonly known as stackers. With such stackers a drum is vertically arranged and a conveyor belt is wound around the drum. The conveyor belt is driven by this drum. With a so-called full stacker the conveyor belt is stacked and supports itself. One portion of the conveyor belt supports the stacked portions on top of this supporting portion. With half stackers one side of the conveyor belt adjacent to the drum supports the stacked portions above this supporting portion and the other side of the conveyor belt is supported by a stationary guide rail.
The first stacker systems used positive driving of the conveyor belt. The drum was provided with for example ribs, which engage with the sides of the conveyor belt providing a shape closed force transfer. Such a driving can result in very high forces in the conveyor belt resulting in early breakage of the conveyor belt.
Another principle for driving the conveyor belt with a drum is the overdrive principle. In such a case a certain slip is maintained between the drum and the conveyor belt. If the forces in the conveyor belt tends to rise, the slip increases, limiting the tension and forces in the conveyor belt. In order to achieve such a slip a drum with a smooth peripheral surface is provided. The drum is driven and mainly drives the conveyor belt. In order to avoid a positive driving of the conveyor belt a second driving means is provided to drive the conveyor belt at a different speed such that slip will occur and be maintained between the drum and the conveyor belt. Especially when starting up such a conveyor assembly it is difficult to control the motor of the drum and the motor of the conveyor belt, such that a slip will occur and be maintained by the driving means.
U.S. Pat. No. 3,348,659 describes a conveyor assembly having a drum around which a conveyor belt is wound. This document does not disclose a stacker belt, but a conveyor belt which is fully supported by guide rails. In order to achieve the necessary slip between the drum and the conveyor belt the document teaches to use a non-cylindrical drum. The drum must decrease in diameter in the transport direction of the conveyor belt. At each turn the diameter of the conveyor belt and the drum decreases such that there is always a speed difference maintained between the peripheral velocity of the drum and the conveyor belt.